Hydrofoil Craft 



Additional, carefully planned, experimentation is needed to provide 

 force, moment, and flutter data which would be useful in connection 

 with the theoretical investigations in progress on aspect ratio, Froude 

 number, and cavitation effects. 



From the structural point of view, we have seen that, though in some 

 cases it was possible to obtain conservative estimates of the flutter 

 speed of swept, low-jU hydrofoils by taking a sufficiently large number 

 of modes, in others the estimates were seriously under-conservative. 

 Though insufficient knowledge of the hydrodynamics may account for 

 much of this discrepancy, there is some evidence that in the low-jU 

 range, additional insight into the problem might be obtained from the 

 application of the differential- equation method of analysis and from 

 correlations of careful observations of flutter mode shapes with 

 theory. In the latter connection, motion pictures appear to be an 

 extremely useful tool. 



Since it appears that many strut-pod-foil systems operate in an effec- 

 tive mass density ratio range which is higher than the asymptotic 

 value, the effects of sweep at moderate sweep angles may be small 

 for such systems, and therefore simplifications in the analysis may 

 be possible. 



As far as actual hydrofoil configurations are concerned, it appears 

 that, where hydroelastic stability is a source of serious concern, 

 scale- model tests are a necessary ingredient in any analysis of the 

 hydroelastic properties of the structure. In this connection, it appears 

 to be a matter of the highest priority to develop techniques for the 

 design and construction of realistic flutter models of strut-pod- 

 hydrofoil prototype configurations. 



Dale Calkins (Naval Undersea Warfare Center, San Diego, Calif.) asked if 

 there were any instances of operational hydrofoils which suffered damage at- 

 tributable to flutter. Tulin felt that the answer had to be no, but probably be- 

 cause of the limited number of craft that have been built and the preponderance 

 of those being of relatively low speed. He went on to emphasize the dramatic 

 failures due to flutter that have been obtained in model scale. 



Next C. Elata (Hydronautics, Inc.) reported upon the principal findings of a 

 study entitled "Choking of Strut-Ventilated Foil Cavities." He concluded that the 

 ventilated cavity would be choked or starved of air for submergence- Froude 

 numbers given by 



F^ < 5 



where 



Cp is the drag coefficient of the foil, 

 A is the foil area, 



1604 



